Came across this in the My Optus app under "offers"
3 months Free - No Contract, month to month data sim, $0 first 3 months then reverts back to the standard $50 (based on the 150gb data sim plan)
The offer is in the MyOptus App which will open web browser. from there select data sim and sign up to the plan you want. it would make the most sense to choose the 150g data plan since all the plans are all free.
You can try the link i have listed however have found that it does not always work as it may be encoded to customer reference. I have shared with someone else and they were able to access the page, however if they refreshed that page a message came up saying "to check if eligible for this offer check the MyOptus App" so if using the link don't refresh. (If you have the app you might be better off with the app)
Checked the critical information and terms and conditions, could not find anything about a contract term. Critial summary says
Optus Choice Data SIM Plan – Month-to-month
3 Months Free: New or existing services who purchase or recontract via the My Optus App to 1. a tablet or watch on a device payment plan with an Optus Choice Data SIM Plan or Optus Choice Connected Watch Plan or 2. An Optus Choice Data SIM plan for three months . N/A with other discount offers unless specified. Ends 30/06/21.